Electron Energy-Loss Spectroscopy, EELS, is typically practiced on Transmission Electron Microscopes (TEM) at 200-300 kV operating voltage and around 1 eV energy resolution. Recently there has been a drive to much lower operating voltage (e.g. 15 kV) and higher energy resolution (e.g. 5 meV). The electron energy-loss spectrum extends from 0 to many keVs energy-loss. The intensity of the energy-loss spectrum falls off rapidly with energy loss, and EELS studies are typically limited to about 3 keV energy loss. This range is expected to increase in the future with improvements in detector technology.
With reference to FIG. 1, in the typical electron energy loss spectrometer an electron beam 5 emerges from a transmission electron microscope 1 and enters the spectrometer through an aperture 10. A bending magnet 13 bends the electron beam over an angle, typically 90 degrees, to introduce energy dispersion. The resulting energy-loss spectrum is electron optically magnified, focused and projected by lenses 15, 16, 17 on a purpose-designed electron detector 21. Focusing is important in the dispersion direction, the spectrum is not necessarily focused in the non-dispersion direction. The detector records what is effectively a window on the energy-loss spectrum. This window has a starting and an ending energy-loss. The starting energy loss is typically set by adjusting the TEM high voltage, the magnetic field of the bending magnet, or the voltage on an isolated drift tube 14 through the bending magnets. Adjusting any of these moves the whole energy-loss spectrum relative to the detector in order to view different regions of interest of the spectrum. The drift tube method is normally preferred as it is very fast (under 1 ms is possible) accurate (under 1 meV is possible) and repeatable (under 1 meV is possible). The width of the recorded window in eV follows from the energy dispersion in eV/m at the detector. Smaller dispersions give larger fields of view in eV, larger dispersions smaller fields of view. The dispersion is controlled by changing the electron optical lenses 16, 17 between the bending magnet 13 and the detector 21. Changing the dispersion, effectively changing the magnification, allows one to zoom in and out of particular spectral details. The electron optics typically uses magnetic quadrupoles for the control of the first order properties such as focus and magnification, as well as a combination of magnetic sextupoles, octupoles, decapoles and dodecapoles to minimize various electron-optical aberrations introduced by the bending magnet and the post bending magnet optics 16, 17. One could use electrostatic instead of magnetic multipoles.
At lower operating voltages and higher energy resolutions, two problems become particularly noticeable, whenever the bending magnet drift tube is employed to shift the spectrum. First, a lens effect occurs at the start and end of the drift tube. Second, because the range of electron energies through the rest of the spectrometer optics has now changed, that optics is somewhat out of focus. These problems cause the EEL spectrum to defocus slightly, limiting energy resolution and spectrum quality.
The portions of the electron energy-loss spectrum that fall outside the field of view recorded by the detector must be trapped carefully so that the electrons do not scatter inside the spectrometer and hit the detector. In practice, this is very difficult and EEL spectrometers typically suffer from what are called reflections. Here the leading and trailing sections of the energy loss spectrum scatter off the drift tube walls causing possible artifacts within the spectrum window as recorded by the detector. Other locations and forms of scattering are possible too. The artifacts can obscure spectral features, make their analysis impossible, and/or be wrongfully interpreted as spectral information itself. In the prior art, scattered electrons are typically intercepted by adding baffles 18 that prevent them from reaching the detector. While reasonably effective this does not stop all scattered electrons and some artifacts typically remain.